"Except for section 6103 (o)(1), which authorizes the discloser of tax information to Federal Employee whose official duties require such information"

Would the Honorable Judge Clevert have proceeded if he found out that ATF at the same time, in another part of the country declared an AR-15 with M-16 parts [including bolt carrier] was NOT a machine gun, and was removed from the National Machine gun Registry?

I doubt it. I also doubt the jury would have bought the ATF fictional display on the stand if they had access to it. I also believe it would never have gone to court if they knew they had to disclose it and the jury would see it.